1. Field of the Invention
The invention generally relates to lasers such as excimer lasers and, in particular, relates to the calibration of lasers and the detection of wavelength drift within such lasers.
2. Description of Related Art
Lasers such as excimer lasers are commonly employed in a wide variety of applications. Often, the output wavelength or frequency of the excimer laser must be precisely regulated to be substantially constant as a function of time and operating conditions. One such application which requires precise wavelength regulation is the use of excimer lasers in the fabrication of integrated circuits.
Conventional techniques for fabricating integrated circuits use a laser beam generated by an excimer laser to harden an optically sensitive material. Portions of the material not hardened by the laser beam are subsequently etched away. Additional fabrication steps are employed to achieve a circuit matching the shape of the hardened material. The short wavelength of an excimer laser operating in the deep ultraviolet region provides the potential for very sharp edge definition. However, when hardening the optically sensitive material using the laser beam from the excimer laser, the wavelength of the laser beam must be precisely controlled to match the wave length for which the optics of the system were designed and setup. A variation in the wavelength of the laser beam can cause the beam to be defocused at the surface of the optically sensitive layer, thereby loosing the advantage of the short wave length and resulting in an imprecise rendering of the integrated circuit. This can adversely affect the electrical characteristics of the resulting integrated circuit, resulting in a poor quality or inoperable circuit. Accordingly, precise regulation of the output wavelength of the excimer laser is critical in the fabrication of the integrated circuits. Other applications using excimer lasers also require precise control of the wavelength of the laser.
U.S. Pat. No. 4,959,840, "Compact Excimer Laser Including an Electrode Mounted in Insulating Relationship to Wall of the Laser", assigned to Cymer Laser Technologies, assignee of the present application, describes a pulsed excimer laser suitable for the use in the fabrication of integrated circuits. U.S. Pat. No. 4,959,840 is incorporated by reference herein, and is referred to hereinafter as the "'840 patent".
U.S. Pat. No. 5,025,445 entitled "System for, and Method of, Regulating the Wavelength of a Light Beam", also assigned to Cymer Laser Technologies, provides a wave-meter apparatus for use with an excimer laser for precisely regulating the wavelength output from the excimer laser. The wavelength regulation technique of U.S. Pat. No. 5,025,445 is appropriate for use in controlling the output wavelength of the excimer laser of the '840 patent as well as other lasers. U.S. Pat. No. 5,025,445 is also incorporated by reference herein and is referred to hereinafter as the "'445 patent".
The wavelength regulation method and apparatus described in the '445 patent provides an effective technique for precisely regulating the wavelength of an excimer laser. In one embodiment of the invention set forth in the '445 patent, a laser light beam is processed in a first optical path to produce light indications in a plurality of free spectral paths. The light indications are introduced through slits to produce signals at spaced positions at opposite peripheral ends of a linear detector array. The distances between correlated pairs of energized detectors being indicate the relative value of the laser wavelength in the free spectral ranges. Spectral laser light beam is also processed in the second optical path simultaneously with the processor beam of the first optical path, to produce light in a single path. The second optical path is dependent on the wavelength laser light beam. The light produced in the second optical path may be introduced through another slit to energize centrally disposed detectors in the array. The particular detectors energized are dependent upon the wavelength of the laser light. The detectors in the linear array is scanned to produce signals related in time to the disposition of the detectors energized in the array. A data processing system processes the signals and produces a signal to adjust the wavelength of the laser beam to a particular value.
The wavelength regulation technique of the '445 patent operates to precisely regulate the wavelength of the laser beam. However, the accuracy of the technique depends upon an initial calibration of the wavemeter system. If improperly calibrated, the wavemeter will operate to precisely regulate the wavelength of the laser to a wavelength offset from an intended wavelength.
Several conventional techniques may be employed for calibrating the output of a laser. These conventional techniques may be employed for calibrating a laser fitted with the wave-meter of the '445 patent. One such conventional calibration technique employs an opto-galvanic sensor. A typical opto-galvanic sensor includes a hollow cathode lamp positioned within a path of a laser beam. The hollow cathode lamp is filled with a selected filler gas such as hydrogen, helium, neon, or mixtures of argon and neon, krypton and neon, or xenon and neon. An anode and a cathode are mounted within the lamp with the cathode having an annular or tubular shape. The cathode is aligned with the laser beam whereby the laser beam passes through an interior of the cathode without touching the cathode material. The cathode is fabricated from any of a large number of elemental materials such as silver, aluminum, gold, iron, or zirconium. In use, an electrical current is conducted through the anode and cathode causing the filler gas within the cathode to be ionized and causing a portion of the material of the cathode to vaporize. The ionized gas and the vaporous cathode material form a plasma in the interior of the cathode within the path of the laser beam. For certain wavelengths of the laser, and depending upon the composition of the filler gas and the cathode material, a resonance can occur between the plasma and the incident laser beam. When the wavelength of the laser is resonance with certain absorption wavelengths of atoms and molecules within the plasma, electrical properties of the plasma are altered. This phenomena is generally referred to as the opto-galvanic effect. The resonance within the plasma affects electrical properties of the current conducted through the anode and cathode. These electrical properties are detected and correlated with the laser beam to yield a determination of the wavelength of the laser beam. Typically, the wavelength of the laser beam is adjusted to achieve a maximum resonance, with the correct wavelength, corresponding to the maximum resonance, being predetermined from the composition of the filler gas and cathode materials. Hence, knowledge of the resonance wavelength may be used to calibrate the laser beam. Typically, a specific combination of cathode material and filler gas is chosen to provide a resonance wavelength in a vicinity of an intended operational wavelength of the laser. For example, by selecting silver as the cathode material and helium as the filler gas material, a certain wavelength of maximum resonance is achieved against which the output of the laser is calibrated.
An exemplary opto-galvanic sensor is illustrated in FIGS. 1A and 1B. Opto-galvanic sensor 10 includes a transparent vacuum tube 12 having an entrance window 14 and an exit window 16. An anode 18 and a cathode 20 are mounted therein. A laser beam, identified by reference numeral 22, enters entrance window 12, passes through the interior of cathode 20 and exits through exit window 16. Circuitry for powering sensor 10, detecting a resonance effect within sensor 10 and adjusting the wavelength of the laser beam to achieve a maximum resonance are not fully shown in FIG. 1. Although laser beam 22 is illustrated with a single narrow line, it should be understood that the beam actually has a width, which may be almost equal to an internal diameter to cathode 20.
Although the opto-galvanic resonance effect has been used somewhat effectively in calibrating lasers, there are several disadvantages inherent in the opto-galvanic technique. A first disadvantage is that alignment of the laser beam and the opto-galvanic sensor is critical. Care must be taken to ensure that the laser beam does not directly strike the cathode material, otherwise substantial photo-electric noise may be generated, obscuring the desired electrical signals. Precise alignment is particularly difficult to achieve with laser beams having fairly broad beam widths or waists. Indeed, for laser beams having a beam width greater than an interior diameter of the cathode, alignment cannot be achieved and the beam will necessarily strike portions of the cathode. Such is a particular problem when a cliffuser is placed in the laser beam, as the diffuser broadens the width of the beam. Such diffusers are commonly employed with an excimer laser, such as the one described in the '840 patent, to eliminate wavelength variations across the beam thereby achieving a beam having uniform wavelength. Hence, for systems employing a diffuser within the path of the laser beam, an opto-galvanic sensor employing the opto-galvanic resonance effect sometimes cannot effectively be used as a calibration tool. Even where the width of the laser beam is somewhat less than the internal diameter of the cathode, alignment may be difficult.
Another disadvantage of calibrating a laser beam using the opto-galvanic effect, is that considerable laser power is required to generate a resonance condition. In many circumstances the laser beam employed for calibration does not provide sufficient intensity to allow for calibration using the opto-galvanic effect. Even for lasers which are capable of producing a sufficiently intense beam, it is often desirable to calibrate the laser without using a high output power level.
Accordingly, although a calibration technique which exploits the opto-galvanic resonance effect to determine the wavelength of a laser beam is effective for some applications, it is ineffective for others. It is desirable to provide an alternative calibration technique which determines the output wavelength of a laser beam even where a relatively low power beam is employed or where the laser beam is wide relative to the cathode of an opto-galvanic sensor.